Speaker
Description
Stacked layers of copper-phthalocyanine (CuPc) and PTCDA on Ag(111) represent a structurally well-defined model for an organic donor-acceptor interface above a metal substrate. We investigate the excitation dynamics of this system by means of two-photon photoemission (2PPE). Besides direct 2PPE from the CuPc HOMO and photoemission involving the n=1 and n=2 image potential states (IPS), we observe an unoccupied Shockley-derived interface state (IS) at the PTCDA / Ag(111) interface. Photoemission from the IS for varying pump photon energies ranging from 1.6 eV to 2.5 eV mainly reflects the optical absorption spectrum of CuPc. This indicates efficient transfer of photoexcited electrons from the CuPc layer to the IS at the PTCDA / silver interface. Using time-resolved 2PPE we are able to follow the dynamics of this charge transfer process which takes place on a femtosecond timescale. Moreover, we discuss the observation of excitonic bands in the 2PPE spectra and investigate the relaxation dynamics of excitonic states in the CuPc / PTCDA donor-acceptor type heterosystem.
